Fuel Cell Anode Structures For Voltage Reversal Tolerance

ABSTRACT

A voltage reversal tolerant fuel cell anode structure that includes a gas diffusion layer is prepared by a method that comprises: (a) applying to the gas diffusion layer a first carbon component comprising a sacrificial carbon component having substantially no resistance to corrosion during cell reversal at fuel cell operating temperatures, and (b) applying to the gas diffusion layer a second carbon component. The first carbon material has a BET surface area of at least 350 m 2 g −1 . The second carbon component supports an electrocatalyst material and has substantially more resistance to corrosion during cell reversal at fuel cell operating temperatures than the first carbon component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/781,234 filed Feb. 18, 2004, entitled “Fuel Cell Anode Structures ForVoltage Reversal Tolerance”. The '234 application is, in turn, acontinuation of U.S. patent application Ser. No. 09/835,905 filed Apr.16, 2001. The '905 application is, in turn, a continuation-in-part ofU.S. patent application Ser. No. 09/585,696 filed Jun. 1, 2000 (now U.S.Pat. No. 6,517,962 issued Feb. 11, 2003). The '905 application is alsorelated to and claimed priority benefits from PCT/InternationalApplication No. PCT/GB01/00458 filed Feb. 6, 2001. The '696 applicationis, in turn, related to and claimed priority benefits from U.S.Provisional Patent Application Ser. No. 60/150,253 filed Aug. 23, 1999.Each of the '234, '905, '696, '253 and '458 applications is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to an anode structure comprising asubstrate and a first carbon-based component that shows little or noresistance to corrosion, such that when the anode structure isincorporated into a membrane electrode assembly, the membrane electrodeassembly is substantially tolerant to incidences of cell voltagereversal.

BACKGROUND OF THE INVENTION

A fuel cell is an energy conversion device that efficiently convertschemical energy into electrical energy by electrochemically combiningeither hydrogen, normally stored as a gas, or methanol, normally storedas a liquid or gas, with oxygen, normally in the form of air, togenerate electrical power. At their fundamental level, fuel cellscomprise electrochemical cells formed from a solid or liquid electrolyteand two electrodes, the anode side and cathode side, at which thedesired electrochemical reactions take place. In the fuel cell, thehydrogen or methanol is oxidized at the anode side and the oxygen isreduced at the cathode side to generate the electrical power.

Normally in fuel cells the reactants are in gaseous form and arediffused into the anode and cathode structures. The electrode structuresare therefore specifically designed to be porous to gas diffusion inorder to facilitate contact between the reactants and the reaction sitesin the electrode to promote the reaction rate. Efficient removal of thereaction products from the electrode structures is also important. Incases where liquid reactants and products are present the electrodestructures are often tailored to efficiently feed reactants to andremove products from the reaction sites. The electrolyte is also incontact with both electrodes and in fuel cell devices may be acidic oralkaline, as well as liquid or solid in nature.

The proton exchange membrane fuel cell (PEMFC) is the likely type offuel cell to find wide application as an efficient and low emissionpower generation technology for a range of markets, such as in a rangeof stationary, residential and portable power generation devices and asan alternative to the internal combustion engine for transportation. Inthe PEMFC, whether hydrogen or methanol fuelled, the electrolyte is asolid proton-conducting polymer membrane, commonly based onperfluorosulfonic acid materials.

In the PEMFC, the combined laminate structure formed from the membraneand the two electrode structures is known as a membrane electrodeassembly (MEA). The MEA typically comprises several layers, but ingeneral can be considered to comprise five layers that are characterizedby their function. On either side of the membrane an anodeelectrocatalyst or a cathode electrocatalyst is incorporated to increasethe rates of the desired electrode reactions. In contact with theelectrocatalyst containing layers, on the opposite face to that incontact with the membrane, are the anode and cathode gas diffusionlayers. The anode gas diffusion layer is typically porous to allow thereactant hydrogen or methanol to enter from the face of the layerexposed to the reactant fuel supply. The reactant then diffuses throughthe thickness of the gas diffusion layer to the layer containing theelectrocatalyst, which is usually platinum metal based, to facilitatethe electrochemical oxidation of hydrogen or methanol. The anodeelectrocatalyst layer also typically comprises some level ofproton-conducting electrolyte in contact with the same electrocatalystreaction sites. With acidic electrolyte types, the product of the anodereaction is protons, and the protons are then transported from the anodereaction sites through the electrolyte to the cathode layers. Thecathode gas diffusion layer is also typically porous to allow oxygen orair to enter the layer and diffuse through to the electrocatalyst layerreaction sites. The cathode electrocatalyst facilitates the chemicalcombination of the protons with oxygen to produce water, and alsotypically comprises some level of the proton-conducting electrolyte incontact with the same electrocatalyst reaction sites. Product water thendiffuses out of the cathode structure. The structure of the cathode isnormally designed to enable efficient removal of product water. If waterbuilds up at or in the cathode, it becomes more difficult for thereactant oxygen to diffuse to the reactant sites, and thus theperformance of the fuel cell deteriorates. In the case ofmethanol-fuelled PEMFCs, additional water is present due to the watercontained in the methanol, which can be transported through the membranefrom the anode to the cathode side. The increased quantity of water atthe cathode requires additional water removal capabilities. However, itis also the case with proton-conducting membrane electrolytes, that iftoo much water is removed from the cathode structure, the membrane candry out, thereby resulting in a significant decrease in the performanceof the fuel cell.

The complete MEA can be constructed by several methods. Theelectrocatalyst layers can be bonded to one surface of the gas diffusionlayer to form what is known as a catalyzed gas diffusion layer or gasdiffusion electrode. Two gas diffusion electrodes can be combined withthe solid proton-conducting membrane to form the MEA. Alternatively, twoporous uncatalyzed gas diffusion layers can be combined with a solidproton-conducting polymer membrane that is catalyzed on both sides toform the MEA. Further, one gas diffusion electrode can be combined withone uncatalyzed gas diffusion layer and a solid proton-conductingpolymer membrane that is catalyzed on the side facing the gas diffusionlayer to form the MEA.

The materials typically employed in the fabrication of the uncatalyzedgas diffusion layers of the MEA comprise high density materials such asrigid carbon fiber paper (such as, for example, Toray TGP-H-60 orTGP-H-90 from Toray Industries, Japan) or woven carbon cloths (such asZoltek PWB-3 from Zoltek Corporation, 3101 McKelvey Road, St. Louis,Mo., USA 63044). Layers such as these are usually modified with aparticulate material either embedded within the fiber network or coatedon to the large planar surfaces, or a combination of both. Typically,these particulate materials comprise a carbon black and polymer mix. Theparticulate carbon black material is, for example, an oil furnace black(such as Vulcan XC72R from Cabot Chemicals, Billerica, Ma, USA) or anacetylene black (such as Shawinigan from Chevron Chemicals, Houston,Tex., USA). The polymer most frequently employed ispolytetrafluoroethylene (PTFE). The coating, or embedding, is carriedout in order to improve the water management properties, improve gasdiffusion characteristics, to provide a continuous surface on which toapply the catalyst layer and to improve the electrical conductivity.More recently, electrode structures based on gas diffusion layerscomprising a non-woven network of carbon fibers (carbon fiber structuressuch as Optimat 203 from Technical Fiber Products, Kendal, Cumbria, UK)with a particulate material embedded within the fiber network, asdisclosed in European Patent Publication No. 0791974, have showncomparable performances to structures based on carbon fiber paper orcloth.

The electrocatalyst materials for the anode and cathode structurestypically comprise precious metals, in particular platinum, as thesehave been found to be the most efficient and stable electrocatalysts forlow-temperature fuel cells such as the PEMFC. Platinum is employedeither on its own as the only electrocatalytic metal or in combinationwith other precious metals or base metals. The platinum-basedelectrocatalyst is provided as very small particles (approximately 20-50Å in diameter) of high surface area, which are usually distributed onand supported by larger macroscopic conducting carbon particles toprovide a desired catalyst loading. Conducting carbons are the preferredmaterials to support the catalyst. Particulate carbon black materialstypically employed include Vulcan XC72R and Shawinigan. It is alsopossible to employ a platinum-based electrocatalyst that does notincorporate a support, and in this case it is referred to as anunsupported Pt electrocatalyst.

Each MEA in the PEMFC is sandwiched between electrically conducting flowfield plates that are conventionally based upon carbon and containchannels that feed the MEA with the reactants and through which theproducts are removed. Since each MEA typically delivers 0.6-0.7 V,usually between 10 to 100 such MEAs are each interposed between flowfield plates to form stacks. These stacks are combined electrically inseries or parallel to produce the desired power output for a givenapplication.

Recently, it has been observed that during prolonged operation somecells in large stacks can go into an undesired condition known as cellvoltage reversal or, simply, cell reversal. This is shown by the cellpotential becoming negative rather than the positive value associatedwith normal PEMFC operation. Such cell reversals can be due to depletionin the concentration of the reactants at the cathode or anode sides,which can be caused by a number of factors such as restricted gas flowdue to blocked flow fields or poor water distribution in the MEA. Incombination with this, especially in situations in which a fast dynamicresponse is required, such as in transportation applications, it ispossible that the gas flow cannot respond quickly enough to sustain thecurrent demand. Further, if one cell in a stack shows cell reversal,adjacent cells in the stack may also overheat, resulting in cellreversal.

If the cell reversal is due to a restricted oxygen concentration at theelectrocatalyst sites in the cathode then, to sustain the flow ofcurrent, hydrogen is produced at the cathode,

2H⁺+2e ⁻→H₂

Since hydrogen production at the cathode is very facile at theplatinum-based electrocatalysts typically employed, the electrodepotential is usually only slightly more negative than that for hydrogenoxidation at the anode. The result is that at normal operating currentdensities the cell voltage is normally slightly negative, for example,−0.1 V. This type of cell reversal raises safety and durabilityconcerns, since hydrogen is being produced in the oxidant side of thecell, a significant quantity of heat is generated, and water is nolonger being produced at the cathode. Such product water helps tosustain membrane hydration, especially at the membrane-anode interface,since it promotes the back-diffusion of water.

A major problem occurs, however, if the hydrogen concentration isrestricted at the anode side. In this case to sustain the flow ofcurrent water electrolysis and carbon corrosion can occur, as follows:

2H₂O→O₂+4H⁺+4e ⁻

C+2H₂O→CO₂4H⁺+4e ⁻

Since both electrode reactions occur at more positive electrodepotentials than oxygen reduction at the cathode, again, the cell voltageis negative, but in this case the cell voltage may be as high as −0.8 Vat typical operating current densities. While carbon corrosion isfavored over water electrolysis thermodynamically, the electrochemicalkinetics of water electrolysis are sufficiently facile at theplatinum-based electrocatalysts typically employed in the PEMFC thatinitially water electrolysis principally sustains the current. There isonly a small contribution from corrosion of the carbon components in theanode to the cell current. If, however, the anode catalyst becomesdeactivated for water electrolysis or if the water concentration at theelectrocatalyst sites in the anode becomes significantly depleted, thewater electrolysis current is gradually replaced by increased rates ofcarbon corrosion. In the case of carbon corrosion, water need only bepresent in the vicinity of the relevant, abundant carbon surfaces.During this period the cell voltage becomes more negative (that is, theanode potential becomes more positive) to provide the necessary drivingforce for carbon corrosion. This in turn increases the driving force forthe water electrolysis reaction. The result, if such cell reversal isprolonged, may be irreversible damage to the membrane and catalystlayers due to excessive dehydration and localized heating. Further, thecatalyst carbon support in the anode structure corrodes, with eventualdissolution of the platinum-based catalyst from the support, and theanode gas diffusion layer may become degraded due to corrosion of thecarbon present in the gas diffusion layer structure. In cases where thebipolar flow field plates are based upon carbon the anode flow fieldplate may also be subjected to significant carbon corrosion, therebyresulting in surface pitting and damage to the flow field pattern.

It would therefore be a significant advantage to protect the MEA fromthe effects of cell reversal should a cell go into cell reversal.

SUMMARY OF THE INVENTION

An anode structure for a proton exchange membrane fuel cell (PEMFC)comprises a substrate and a first carbon-based component comprising afirst carbon material. The first carbon-based component exhibits littleor no resistance to corrosion. When the present anode structure isincorporated into a membrane electrode assembly, the MEA issubstantially tolerant to incidences of cell reversal.

The term “anode structure” in the context of the present specificationmeans any of the functional components and structures associated withthe anode side of the MEA through which a fuel is either transported orreacted, that is, within the gas diffusion and electrocatalystcontaining layers on the anode side of the membrane. The practicalembodiments of the present anode structure as herein defined include:

-   -   (a) a gas diffusion layer;    -   (b) an electrocatalyst containing layer bonded to a gas        diffusion layer (also referred to as a gas diffusion electrode        or a catalyst-coated gas diffusion layer)    -   (c) an electrocatalyst containing layer bonded to the        proton-conducting membrane (also referred to as a        catalyst-coated membrane)

In the context of the present specification, the term “substrate” refersto a gas diffusion layer or a polymer membrane electrolyte.

The first carbon-based of the present anode structure component mayconsist entirely of a first carbon material or may comprise a firstcarbon material and one or more other materials that may for example bepresent to promote the corrosion rate of the first carbon material or toact as a binder. The one or more other materials that may be present inthe first carbon-based component include polymeric materials such as,for example, a proton-conducting polymer electrolyte, such as Nafion®,or a non-proton-conducting polymer such as, for example,polytetrafluoroethylene (PTFE). The first carbon-based component presentin the anode structure (whether solely of first carbon material or offirst carbon material plus other material(s)) shows little or noresistance to corrosion, and therefore when used in an electrochemicalcell that has entered a period of cell reversal, the first carbon-basedcomponent will be corroded in preference to other carbon also present inthe anode structure, for example a carbon support for theelectrocatalyst. In other words, the first carbon-based component isacting as a sacrificial carbon component. This will protect furthercarbon present in the anode from corrosion and thus maintain its desiredfunction when the cell returns to normal operation. For instance, thiswill inhibit the carbon black in the electrocatalyst carbon support andthe carbon in the gas diffusion layer from corroding. Consequently, theanode electrocatalyst and the anode gas diffusion layer will beprotected from the effects of cell reversal, thereby allowing the cellto function without having suffered significant irreversible performancedecay when the cell reverts to normal fuel cell operation after the cellreversal incident. To promote the corrosion rate of the first carbonmaterial used in the first carbon-based component, the first carbonmaterial may be pre-treated with a suitable form of theproton-conducting membrane electrolyte prior to incorporation into theanode structure. Impregnating the first carbon material withproton-conducting membrane electrolyte will promote the corrosion rateof the first carbon-based component by providing an efficient conductionpathway for the protons formed in the carbon corrosion reaction to themembrane of the MEA.

Further, the first carbon-based component allows the membrane andcatalyst layer in the MEA to function without having sufferedsignificant irreversible performance decay when the cell reverts tonormal fuel cell operation after the cell reversal incident. This isbecause corrosion of the first carbon-based component helps sustain thecurrent density at a less negative cell voltage, corresponding to a lesspositive anode potential. At less positive anode potentials the drivingforce for irreversible damage to the membrane and catalyst layers isreduced.

As a general rule, the corrosion resistance of carbons is related to thedegree of the graphitic nature within the structure. The more graphiticthe structure of the carbon the more resistant the carbon is tocorrosion. The typical carbon blacks employed in fuel cells, either asthe electrocatalyst support or in the gas diffusion layer, thereforetend to be those that are more highly graphitic in nature as theenvironment particularly at the cathode is very oxidizing. It iscontemplated that the first carbon material will be chosen from thegroup of carbons that are much less graphitic, that is, more amorphousthan the typical carbon materials employed in the fuel cell.

In a further embodiment of the present anode structure, the anodestructure further comprises a second carbon component that issubstantially more resistant to corrosion than the first carbon-basedcomponent. For example, the second carbon component may be a carbonsupport for an electrocatalyst or a carbon fill for a gas diffusionsubstrate.

In embodiments of the present anode structure, a gas diffusion layer maycomprise a first carbon-based component. The first carbon-basedcomponent may either be embedded within the gas diffusion layer orapplied as a coating to one or both surfaces, or a mixture of both. Toprepare a gas diffusion layer according to the present technique, thefirst carbon-based component may be mixed with a carbon black fillermaterial typically employed to coat or fill the carbon paper, cloth ornon-woven fiber web substrates employed in the PEMFC to produce theanode structure of the invention in the form of a gas diffusion layer.To promote the corrosion rate of the first carbon material used in thefirst carbon-based component, the first carbon material may bepre-treated or the resultant anode gas diffusion layer subsequentlytreated with a suitable form of the proton-conducting membraneelectrolyte prior to incorporation in the MEA. The carbon black fillermaterial usually comprises a particulate carbon and a polymer, thecarbon suitably being in the form of a powder. The carbon powder may beany of the materials generally designated as carbon black, such asacetylene blacks, furnace blacks, pitch coke based powders andgraphitized versions of such materials. Suitably, both natural andsynthetic graphites may also be employed in this application. Suchmaterials may be employed either alone or in combination. Theparticulate carbon, or carbons, in the fill are held together by one ormore polymers. The polymeric materials employed contribute to theelectrode structural properties, such as pore size distribution,hydrophobic/hydrophilic balance and physical strength of the gasdiffusion layer. Examples of such polymers include PTFE, fluorinatedethylene-propylene (FEP), polyvinylidene difluoride (PVDF), Viton A,polyethylene, polypropylene, ethylene-propylene. The preferred polymeris PTFE or FEP.

In addition other modifier materials and catalyst materials, which arenot electro-catalysts, may be added to the carbon black filler such asdisclosed in PCT/International Publication No. WO 00/55933 (JohnsonMatthey).

Furthermore, the first carbon-based component may be applied to an anodegas diffusion layer that has previously been coated or filled withtypical carbon filler materials. To promote the corrosion rate of thefirst carbon material employed in the first carbon-based component, itmay be pre-treated with the suitable form of the proton-conductingmembrane electrolyte. It is contemplated that in the MEA formed usingthe resultant anode gas diffusion layer, the first carbon-basedcomponent within the anode layer may face either the electrocatalystlayer or the anode flow field plate. In the present anode structure, theanode gas diffusion layer should have sufficient electrical conductivitysuch that on removal of the first carbon-based component during cellreversal, the remaining anode layer does not have a significantly lowerelectrical conductivity. Typical substrates that could be employedinclude those based upon Toray carbon fiber paper and Zoltek PWB-3carbon cloth, which without a carbon coating or fill have through planespecific electrical resistivities of below 0.15 Ωcm.

In another embodiment of the anode structure, a gas diffusion electrodecomprises a first carbon-based component. The first carbon-basedcomponent may be admixed with an electro-catalyst component and apolymeric material and the two applied to a gas diffusion layer as asingle admixed layer, or the first carbon-based component and theelectrocatalyst component may be applied as separate layers, eachseparate layer also incorporating a polymeric material. Alternatively,there could be a combination of separate and mixed layers. The polymericmaterial may be a soluble form of the proton-conducting membraneelectrolyte, or may be any of a wide range of polymeric materials usedto contribute to the structural and diffusional properties. Examples ofsuch polymers include PTFE, FEP, PVDF, Viton A, polyethylene,polypropylene, ethylene-propylene. The preferred polymer is PTFE or FEP.To promote the corrosion rate of the first carbon material used in thefirst carbon-based component the first carbon material may bepre-treated with a suitable form of the proton-conducting membraneelectrolyte prior to incorporation into the anode electro-catalystmixture.

The mixture of first carbon-based component and anode electrocatalystcan be deposited onto the typical range of gas diffusion layers employedin PEMFCs to produce the anode structure of the invention in the form ofa gas diffusion electrode.

Typical anode electrocatalysts employed in the PEMFC may be, forexample, a precious metal or a transition metal as the metal or metaloxide, either unsupported or supported in a dispersed form on a carbonsupport; an organic complex, in the form of a high surface area finelydivided powder or fiber, or a combination of these options. An exampleof a suitable electrocatalyst material is described in European PatentPublication No. 0731520. Particularly suitable electrocatalysts areunsupported platinum (Pt) or alloys or mixtures of platinum/ruthenium(PtRu) and carbon supported Pt or PtRu. The PtRu electrocatalystexhibits a higher level of tolerance to CO and CO₂ when they are presentin the fuel stream than Pt electrocatalysts.

Specific examples of this embodiment may be prepared according to theprocedure described in more detail below.

In another embodiment of the anode structure, a catalyst-coated membranecomprises a first carbon-based component. The first carbon-basedcomponent may be admixed with an electrocatalyst component and apolymeric material and the two applied to a membrane electrolyte as anadmixed single layer, or the first carbon-based component and theelectrocatalyst component may be applied as separate layers, eachseparate layer also incorporating a polymeric material. Alternatively,there could be a combination of separate and mixed layers. The polymericmaterial may be a soluble form of the proton-conducting membraneelectrolyte, or may be any of a wide range of polymeric materials usedto contribute to the structural and diffusional properties. Examples ofsuch polymers include PTFE, FEP, PVDF, Viton A, polyethylene,polypropylene, ethylene-propylene. The preferred polymer is PTFE or FEP.To promote the corrosion rate of the carbon in the first carboncomponent, the carbon may be pre-treated with a suitable form of theproton-conducting membrane electrolyte prior to incorporation into theanode electrocatalyst mixture.

The mixture of first carbon-based component and anode electrocatalystcan be deposited onto the solid membrane electrolyte to produce theanode structure of the invention in the form of a catalyst-coatedmembrane. Subsequent compression of the anode and cathodecatalyst-coated membrane to the typical gas diffusion layers employed inPEMFCs, or hot pressing of anode and cathode catalyst-coated gasdiffusion layers to the solid proton-conducting membrane electrolyteforms the complete MEA.

Typical anode electrocatalysts employed in the PEMFC are as previouslydescribed.

The proton-conducting polymers suitable for use in the present anodestructure may include, but are not limited to:

-   -   (a) Polymers which have structures with a substantially        fluorinated carbon chain optionally having attached to it side        chains that are substantially fluorinated. These polymers        contain sulfonic acid groups or derivatives of sulfonic acid        groups, carboxylic acid groups or derivatives of carboxylic acid        groups, phosphonic acid groups or derivatives of phosphonic acid        groups, phosphoric acid groups or derivatives of phosphoric acid        groups and/or mixtures of these groups. Perfluorinated polymers        include Nafion®, Flemion® and Aciplex® commercially available        from E. I. DuPont de Nemours (U.S. Pat. Nos. 3,282,875;        4,329,435; 4,330,654; 4,358,545; 4,417,969; 4,610,762; 4,433,082        and 5,094,995), Asahi Glass KK and Asahi Chemical Industry        respectively. Other polymers include those disclosed in U.S.        Pat. No. 5,595,676 (Imperial Chemical Industries plc) and U.S.        Pat. No. 4,940,525 (Dow Chemical Co.).    -   (b) Perfluorinated or partially fluorinated polymers containing        aromatic rings such as those described in PCT/International        Publication Nos. WO 95/08581, WO 95/08581 and WO 97/25369        (Ballard Power Systems Inc.), which have been functionalized        with SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂.        Also included are radiation or chemically grafted perfluorinated        polymers, in which a perfluorinated carbon chain, for example,        PTFE, FEP, tetrafluoroethylene-ethylene (ETFE) copolymers,        tetrafluoroethylene-perfluoroalkoxy (PFA) copolymers, poly        (vinyl fluoride) (PVF) and poly (vinylidene fluoride) (PVDF) is        activated by radiation or chemical initiation in the presence of        a monomer, such as styrene, which can be functionalized to        contain an ion exchange group.    -   (c) Fluorinated polymers such as those disclosed in European        Patent Publication Nos. 0331321 and 0345964 (Imperial Chemical        Industries plc) containing a polymeric chain with pendant        saturated cyclic groups and at least one ion exchange group        which is linked to the polymeric chain through the cyclic group.    -   (d) Aromatic polymers such as those disclosed in European Patent        Publication No. 0574791 and U.S. Pat. No. 5,438,082 (Hoechst        AG), for example sulfonated polyaryletherketone. In addition,        aromatic polymers such as polyether sulfones, which can be        chemically grafted with a polymer with ion exchange        functionality such as those disclosed in PCT/International        Publication No. WO 94/16002 (Allied Signal Inc.).    -   (e) Nonfluorinated polymers include those disclosed in U.S. Pat.        No. 5,468,574 (Dais Corporation), for example, hydrocarbons such        as styrene-(ethylene-butylene)-styrene,        styrene-(ethylene-propylene)-styrene and        acrylonitrile-butadiene-styrene copolymers and terpolymers, in        which the styrene components are functionalized with sulfonate,        phosphoric and/or phosphonic groups.    -   (f) Nitrogen containing polymers including those disclosed in        U.S. Pat. No. 5,599,639 (Hoechst Celanese Corporation), for        example, polybenzimidazole alkyl sulfonic acid and        polybenzimidazole alkyl, or aryl phosphonate.    -   (g) Any of the above polymers which have the ion exchange group        replaced with a sulfonyl chloride (SO₂Cl) or sulfonyl fluoride        (SO₂F) group, thereby rendering the polymers melt processable.        The sulfonyl fluoride polymers may form part of the precursors        to the ion exchange membrane or may be arrived at by subsequent        modification of the ion exchange membrane. The sulfonyl halide        moieties can be converted to a sulfonic acid using conventional        techniques such as, for example, hydrolysis.

In direct methanol fuel cells (DMFC), it is methanol that is oxidized atthe anode during normal fuel cell operation, as follows:

CH₃OH+H₂O

CO₂+6H⁺+6e ⁻

Fuel starvation can also be a particular problem in the methanol-fuelledDMFC. The methanol can be blocked from the electrocatalyst sites by thesignificant quantities of water that are present in the aqueous methanolfuel mixture and by the carbon dioxide gas that is generated by theelectro-oxidation of the methanol. Consequently, the problems of cellreversal due to fuel starvation in the anode structure, which aresubstantially identical to those outlined for the H₂-fuelled PEMFC, canbe a problem in the DMFC. The use of a first carbon-based component inthe anode structure of the DMFC offers a significant benefit. Just as inthe H₂-fuelled PEMFC, the use of a first carbon-based component protectsthe vital carbon components in the anode from corrosion, by undergoingpreferential corrosion, and also protects the membrane and catalystlayers from excessive dehydration and irreversible damage by helping tosustain the current density at less positive anode potentials. The useof a first carbon-based component in the anode structure of the DMFCallows the MEA to provide a performance that is not significantlyreduced after a cell reversal incident. However, the problem of carboncorrosion in the direct methanol fuelled PEM fuel cell is not likely tobe as great a problem as in the H₂-fuel cell due to the increased amountof water at the anode, and thus cell reversal current should be consumedin electrolysis reactions.

In a further aspect, an MEA comprises the present anode structure.

In a still further aspect, a fuel cell comprises an MEA comprising thepresent anode structure. In a yet further aspect, a fuel cell comprisesthe present anode structure.

While the present anode structures have been described for use in solidpolymer fuel cells, such as the proton exchange membrane and directmethanol fuel cells, it is anticipated that they would be useful inother fuel cells, as well. In this regard, “fuel cell” generally refersto a fuel cell having an operating temperature below about 250° C. Thepresent anode structures are preferred for acid electrolyte fuel cells,which are fuel cells comprising a liquid or solid acid electrolyte, suchas phosphoric acid, solid polymer electrolyte, and direct methanol fuelcells. The present anode structures are particularly preferred for solidpolymer electrolyte fuel cells.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

Specific examples of a gas diffusion layer for use in conjunction withthe present anode structure may be prepared in the following manner.

Examples

A particulate catalyst component, containing a first carbon-basedcomponent is provided by dispersing 30 weight parts of a high surfacearea carbon black (the first carbon-based component, which may be Blackpearls 2000 or PICACTIF CSO-D, both available from Cabot Carbon Ltd.,Stanlow, South Wirral, UK, or Norit A Supra, from Norit Americas Inc.,Atlanta, USA) and 100 weight parts of a 20 wt % platinum, 10 wt %ruthenium catalyst, supported on Cabot Vulcan XC72R (from JohnsonMatthey Inc, New Jersey, USA) in 1200 parts of demineralized water. Tothis is added 10 weight parts of polytetrafluoroethylene (PTFE) as adispersion in water (ICI Fluon GP1, 64 wt % solids suspension) and themixture heated and stirred to entrain the PTFE particles within thecarbon catalyst materials. The slurry is filtered to remove excess waterand re-dispersed in a 2% methyl cellulose solution, using a high shearmixer, to produce a smooth mixture.

The anode electrode may be prepared by applying a layer of thecarbon/catalyst slurry described above to a pre-teflonated (18% byweight ICI Fluon GP1), rigid conducting carbon fiber paper substrate(Toray TGP-H-090, available from Toray Industries Inc, Tokyo, Japan) atan electrode platinum loading of 0.4 mg/cm² of electrode geometric area.The dried electrode is heated to 375° C. in air to sinter the PTFE Asolution of perfluorosulfonic acid in the aqueous form as described inEuropean Patent Publication No. 0731520 is applied to the surface of thecatalyst layer to provide a proton conductive interface with theelectrocatalyst and to act as a water reservoir for the carbon corrosionprocess.

An electrode so prepared may form the anode of an MEA. The cathode maybe of the more conventional type, currently widely employed in thePEMFC. The foregoing comprise a conventional pre-teflonated rigidconducting carbon fiber paper substrate (Toray TGP-H-090, available fromToray Industries Inc, Tokyo, Japan) to which is applied a layer of a 40wt % platinum, catalyst, supported on Cabot Vulcan XC72R (from JohnsonMatthey Inc, New Jersey, USA), at an electrode platinum loading of 0.6mg/cm² of electrode geometric area. The catalyst layer material isprovided by dispersing 100 weight parts of a 40 wt % platinum catalyst,supported on carbon black (Johnson Matthey High-Spec 4000) in 30 partsof a 9.5% dispersion of Nafion EW1100 (E.I. DuPont de Nemours & Co.) inwater, prepared according to methods described in EPA 731,520. Theparticulate catalyst is dispersed using a high shear mixer to produce asmooth mixture and is then applied to the cathode substrate. Thecomplete MEA is fabricated by bonding the anode and the cathodeelectrodes (with the face of the electrode comprising the platinumcatalyst component adjacent to the membrane) to a Nafion 112 membrane(supplied by E.I. DuPont de Nemours, Fayetteville, N.C., USA)

The MEA thus formed may be tested in a cell reversal situation accordingto the following procedure. The MEA is conditioned prior to voltagereversal by operating it normally at a current density of about 0.5A/cm² and a temperature of approximately 75° C. Humidified hydrogen maybe used as fuel and humidified air as oxidant, both at 200 kPa pressure.The stoichiometry of the reactants (that is, the ratio of reactantsupplied to reactant consumed in the generation of electricity) is 1.5and 2.0 for the hydrogen and oxygen-containing air reactants,respectively. The output cell voltage as a function of current density(polarization data) is determined. After that, each cell is subjected toa voltage reversal test by flowing humidified nitrogen over the anode(instead of fuel) while forcing 10A current through the cell for aperiod of time long enough to cause some damage to a conventional anodewithout causing the extensive damage associated with large increases inthe anode potential (23 minutes has been found to be an appropriatelength of time) using a constant current power supply connected acrossthe fuel cell. During the voltage reversal, the cell voltage versus timeis recorded. Polarization data for each cell is obtained once the cellhas returned to normal stabilized operating conditions to determine theeffect of a single reversal episode on cell performance.

Each cell is then subjected to a second voltage reversal test at a 10Acurrent. This time, however, the reversal current is interrupted fivetimes during the test period to observe the effect of repeated reversalson the cells. After 5 minutes of operation in reversal, the current iscycled on and off five times (20 seconds off and 10 seconds on) afterwhich the current is left on until a total “on” time of 23 minutes hasbeen reached. Following the second reversal test, polarizationmeasurements of each cell are obtained.

The above procedure for cell testing can be used not only for the threespecific examples described, but also for examples falling within thescope of the present teachings. Furthermore, although the examplesdescribed above relate to a gas diffusion layer according to the presentteachings, it is within the ability of those skilled in the art tomodify the procedure to produce a gas diffusion substrate and/or acatalyst-coated membrane according to the present teachings.

In the foregoing examples and embodiments, the rate of carbon corrosionmay be determined by appropriate adaptation of the following procedurewhich is suitable for a liquid acid electrolyte fuel cell such as, forexample, a phosphoric acid fuel cell. A complete cell is assembled byinserting an anode structure (previously weighed) of the invention and areference electrode (for example, a dynamic hydrogen referenceelectrode) into a liquid electrolyte. The cell was left until the testtemperature is reached (for example, 180° C.) and for the open circuitvoltage (OCV) of the anode structure to stabilize. The cell wasactivated and as soon as the potential of the working electrode reached1 volt, current readings were taken over a given time period. The cellwas dismantled and the anode structure reweighed. The log (corrosioncurrent) was plotted against log(time) and extrapolated to 100 minutes.The corrosion rate is expressed as current per unit weight of carbon(μAmg⁻¹C) after 100 minutes at 1 volt. Data for the corrosion rates of anumber of carbons in phosphoric acid fuel cells may be found inCatalysis Today, 7 (1990) 113-137, which is incorporated herein byreference in its entirety. Although the actual carbon corrosion rateswill be dependent on the particular environment in which the anodestructure is placed, the relative rates of the various carbons willremain substantially similar.

One measure that can be taken as an indication of the corrosionresistance of carbon is provided by the BET surface area measured usingnitrogen, as this detects the microporosity and mesoporosity typicallyfound in amorphous carbon structures. For example, Vulcan XC72R,Shawinigan and graphitized Vulcan XC72R are typicalsemi-graphitic/graphitic carbon blacks employed in fuel cells. VulcanXC72R has a surface area of 228 m²g⁻¹. This contrasts with a surfacearea of 86 m²g⁻¹ for graphitized Vulcan XC72R. The much lower surfacearea as a result of the graphitization process reflects a loss in themore amorphous microporosity in Vulcan XC72R. The microporosity iscommonly defined as the surface area contained in the pores of diameterless than 2 nm. Shawinigan has a surface area of 55 m²g⁻¹, and BETanalysis indicates a low level of carbon microporosity available in thissupport for corrosion. This contrasts with the much higher BET surfacearea of, for example, Black Pearls 2000 (1536 m²g⁻¹) reflecting in thiscase a high degree of microporosity in this carbon black that cancorrode. Carbon blacks with BET surface areas in excess of 350 m²g⁻¹,such as BP2000, could be employed as the first carbon material in thefirst carbon-based component in the anode structure of the PEMFC.

There are other carbons that also have high BET surface areas in excessof 350 m²g⁻¹, such as those classified as activated carbons. Suchcarbons are usually derived from the carbonization of vegetable matter(typically wood, peat or coconut husks), in which the carbon isgenerally amorphous in character and there is a range of possible poresizes from micropores to larger mesopores and macropores. Typicalexamples of these activated carbons are those produced under the generaltrade name Norit (Norit Americas Inc., Atlanta, Ga., USA) and Pica(Pica, 92300 Levallois, France). Such carbons could also be employed asthe first carbon material in the first carbon-based component in theanode structure of the PEMFC.

Another indication of the corrosion resistance may be demonstrated bythe carbon inter-layer separation d₀₀₂ measured from the x-raydiffractograms. Synthetic graphite (substantially pure graphite) has aspacing of 3.36 Å compared with 3.45 Å for Vulcan XC72R (graphitized),3.50 Å for Shawinigan, and 3.64 Å for Vulcan XC72R, with the higherinter-layer separations reflecting the decreasing graphitic nature ofthe carbon and the decreasing order of corrosion resistance. Thus, afirst carbon material with an inter-layer separation of greater that3.65 Å may be suitable for use in the first carbon-based component ofthe present invention. However, many carbons that show poor resistanceto corrosion (and therefore may be of use in the first carbon-basedcomponent of the present invention) are amorphous in nature andtherefore no inter-layer separation measurement can be obtained.

It is also possible that the first carbon-based component may comprise afirst carbon material which intrinsically demonstrates a reasonably highresistance to corrosion but which is treated in such a manner, forexample, by coating with a proton-conducting electrolyte, that theformed first carbon-based component as a whole shows little or noresistance to corrosion.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from the scopeof the present disclosure, particularly in light of the foregoingteachings.

1. A method of preparing a voltage reversal tolerant fuel cell anodestructure comprising a gas diffusion layer, the method comprising: (a)applying to said gas diffusion layer a first carbon component comprisinga sacrificial carbon component having substantially no resistance tocorrosion during cell reversal at fuel cell operating temperatures andsaid first carbon material having a BET surface area of at least 350m²g⁻¹, (b) applying to said gas diffusion layer a second carboncomponent, said second carbon component supporting an electrocatalystmaterial, said second carbon component having substantially moreresistance to corrosion during cell reversal at fuel cell operatingtemperatures than said first carbon component.
 2. The method of claim 1wherein said first carbon component and said second carbon componentsare mixed before applying to said gas diffusion layer.
 3. An improvedmethod of imparting voltage reversal tolerance to a fuel cell anodestructure comprising a gas diffusion layer, said gas diffusion layerhaving an electrocatalytic material disposed on a carbon support appliedthereto, the improvement comprising: applying to said gas diffusionlayer a sacrificial carbon component having substantially no resistanceto corrosion during cell reversal at fuel cell operating temperaturesand having a BET surface area of at least 350 m²g⁻¹.
 4. A method ofpreparing a voltage reversal tolerant fuel cell anode structurecomprising a gas diffusion layer, the method comprising: (a)incorporating into said gas diffusion layer a first carbon componentcomprising a sacrificial carbon component having substantially noresistance to corrosion during cell reversal at fuel cell operatingtemperatures and said first carbon material having a BET surface area ofat least 350 m²g⁻¹, (b) incorporating into said gas diffusion layer asecond carbon component, said second carbon component supporting anelectrocatalyst material, said second carbon component havingsubstantially more resistance to corrosion during cell reversal at fuelcell operating temperatures than said first carbon component.
 5. Themethod of claim 4 wherein said first carbon component and said secondcarbon components are mixed before incorporation into said gas diffusionlayer.
 6. An improved method of imparting voltage reversal tolerance toa fuel cell anode structure comprising a gas diffusion layer, said gasdiffusion layer comprising an electrocatalytic material disposed on acarbon support, the improvement comprising: incorporating into said gasdiffusion layer a sacrificial carbon component having substantially noresistance to corrosion during cell reversal at fuel cell operatingtemperatures and having a BET surface area of at least 350 m²g⁻¹.